The invention relates to a working machine and a method for operating a working machine.
The invention is applicable on working machines within the field of industrial construction machines, in particular wheel loaders. Although the invention will be described with respect to a wheel loader, the invention is not restricted to this particular vehicle, but may also be used in other heavy working machines, such as articulated haulers, trucks and excavators.
A wheel loader is usually provided with an internal combustion engine, a transmission line, and a gearbox for supplying torque to the driving wheels of the wheel loader. The gearbox provides different gear ratios for varying the speed of the vehicle, and for changing between forward and backward driving direction. The transmission line comprises a hydrodynamic torque converter arranged between the internal combustion engine and the gearbox. The torque converter is used to increase the torque during particularly heavy working operations, such as filling the bucket or acceleration of the wheel loader. The torque converter can very quickly adapt the output torque to the current working conditions. However, the torque converter has often a very low efficiency which is also dependent on the current driving conditions. The efficiency can be increased if the torque converter is provided with a lock-up function which can be used for direct operation. However, the gear ratio is fixed (1:1) in the lock-up state, and the problem of low efficiency remains during working operations where such a lock-up function cannot be used.
In addition to supply torque to the driving wheels, the internal combustion engine has to supply energy to a hydraulic pump of a hydraulic system of the wheel loader. Such a hydraulic system is used for lifting operations and/or steering the wheel loader. Hydraulic working cylinders are arranged for lifting and lowering a lifting arm unit, on which a bucket or other type of attachment or working tool, for example forks, is mounted. By use of another hydraulic working cylinder, the bucket can also be tilted or pivoted. Further hydraulic cylinders known as steering cylinders are arranged to turn the wheel loader by means of relative movement of a front and rear body part of the wheel loader.
On one hand, the rotation speed of the internal combustion engine has to be adapted to the hydraulic pump, and on the other hand, the internal combustion engine has to be adapted to the need for a very high torque, for example during filling the bucket when the speed of the wheel loader is close to zero. A rotation speed of the internal combustion engine determined by the hydraulic system means that the wheel loader has to be braked so as to adapt the velocity of the wheel loader to the current conditions. In order to fulfill the demands on speed of rotation and torque in different situations, the internal combustion engine has to be dimensioned for an unnecessary high effective output which output is not required or is only infrequently required. The use of an internal combustion engine which in some respects is oversized and further has to be driven during circumstances where the torque converter has low efficiency, and/or during conditions where the wheel loader has to be braked due to the hydraulic system, will result in high fuel consumption.
It is desirable that the invention is to provide a working machine of the kind referred to in the introduction, which working machine enables a more effective operation of the working machine and lower fuel consumption.
By the provision of a transmission line comprising an electric machine for driving or braking the driving wheels, and/or for generating electric power for the hydraulic pump, the internal combustion engine can be more efficiently used, which enables a smaller internal combustion engine to be used, and the fuel consumption can be lowered. For example, the hydraulic system can be driven at least partly by the electric machine instead of the internal combustion engine. Thus, the rotation speed of the internal combustion engine does not need to be determined taking the hydraulic pump into consideration. The electric machine can be used as a brake during a braking operation of the wheel loader, and at the same time function as a generator for recuperating energy. The energy can be directly supplied to the hydraulic system or stored in an electric energy storage means, such as a battery or super capacitor, to be used later on. If a great traction force is required, an additionally torque can be supplied to the driving wheels by means of the electric machine functioning as a electric motor, and, thus the need of the torque converter is decreased.
Furthermore, the electric machine can be used for driving the driving wheels in the reversed direction. This means that the reverse gear of the gearbox can be eliminated. A further advantage is that the electric machine can be used for adapting the speed of rotation of the internal combustion engine and the speed of rotation of the transmission line to each other so as to facilitate coupling or decoupling of a direct operation state of a transmission unit comprised in the transmission line.
By such a method the size and operation of the internal combustion engine can be optimized so as to keep the fuel consumption as low as possible.
Further advantages and advantageous features of the invention are disclosed in the following description.
By the term “electric machine” is meant a combined electric motor and generator. The electric machine can be driven by electricity to supply an output torque on a shaft or be mechanically driven by applying torque on a shaft for producing electricity.
The term “transmission unit” comprises hydraulic clutches, both hydrodynamic clutches such as torque converters and hydrostatic clutches, as well as mechanical clutches. Thus, “transmission unit” comprises both torque converters which can increase the torque, and ordinary clutches only used for disengagement and direct operation with the gear ratio of 1:1.